Maritime Molluscs in Warming Waters

Over 15,000 species of aquatic bivalves – including clams, scallops, mussels, and oysters – live throughout marine and freshwater ecosystems around the globe. The name “bivalve” is derived from their characteristic two-part shell joined by ligaments and strong adductor muscles.

These tasty shellfish are served on seafood platters in restaurants and enjoyed at family gatherings, but at what point do we take the bivalve community for granted? The increasing effects of climate change will shift the performance of coastal ecosystems, and could pose a threat to the marine-derived food supply and access to it.

Climate change represents the unknown for global fisheries and aquaculture. One researcher and his team set out to find answers. They created scenarios replicating the anticipated effects of climate change on coastal ecosystems and some of their aquatic inhabitants, bivalves.

Mimicking Climate Change

Melting glaciers and rising sea level are the result of the current state of global climate change. Some consequences are visible now, but what else may happen if climate change continues on its steady escalation?

Ecosystem modelling is an approach used to simulate the dynamics and interactions of different ecosystem compartments. Models can be used to evaluate our understanding of ecosystem functioning, determine the current state of an ecosystem, or how it may respond to future unpredictable stressors.

These scenarios took place in a simulated bay that resembled coastal aspects of the Gulf of St. Lawrence, where the two species of bivalves of interest to this study - Crassostrea virginica (the eastern oyster) and Mytilus edulis (the blue mussel) - are found, cultured and harvested.

Temperature is Critical

“Our research highlighted the critical role of temperature over other climate change drivers,” says Dr. Filgueira. This is due to its influence on the metabolism of poikilothermic organisms, those such as mussels and oysters whose internal temperature is highly dependent on the temperature of their surrounding environment.

A key driver of ecosystem functioning and bivalve health is phytoplankton. These microorganisms and producers are at the bottom of the food chain that serve as the main food source for bivalves. Their growth depends on carbon dioxide and water temperature – key variables of climate change.

With increasing water temperatures, the phytoplankton population was shown to decrease due to the feeding activity of zooplankton, who also live off of phytoplankton. However, the levels to which phytoplankton were reduced were also a function of the discharge of riverine nutrients, morphology of the bay, and level of aquaculture development. The results also predicted that under current aquaculture practices, mussel aquaculture puts more pressure on phytoplankton populations than oysters.

In the particular case of the temperature regime of Eastern Canada, and assuming that temperature increase will not reach lethal thresholds for these species, both mussels and oysters will actually benefit from the heat. One reason for this is that the increase in temperature leads to increasing metabolic rates, causing an increase in growth rates, assuming that food availability is not a limiting factor.

“Given the different thermal tolerances of cultured species, future temperature is also critical to sort winners from losers, benefiting the eastern oyster over the blue mussel,” says Dr, Filgueira.

Each species has a different tolerance to water temperature, and oysters, which naturally occupies warmer habitats than mussels, will benefit from the temperature increase in the Gulf of St. Lawrence, particularly in the winter months. On the other hand, mussels could experience decreased growth and potentially death if summer temperatures reach optimal and lethal thresholds, respectively. The thresholds for mussels could be reached in the near future according to some climate change scenarios.

Survival of the Fittest

The results of Dr. Filgueira’s research indicated that oysters are more likely to thrive over mussels in embayments of the Gulf of St. Lawrence under the pressures of climate change drivers, such as high precipitation and increasing sea temperatures.

Temperature as a driver of climate change wasn’t the only factor in bivalve performance. Physical environment was also used to measure the failure or success of these shelled seafood.

The models used mimicked both oceans and rivers, the later representing a typical coastline in Prince Edward Island. Filgueira found that “…bays with large rivers and high exchange with the open ocean will be more resilient under climate change when bivalve aquaculture is present.”. This is related to high riverine nutrient discharge and water renewal in these systems, which minimizes the potential negative impacts of cultured bivalves on phytoplankton populations.

“Different environments are responding in different ways. Climate change effects are specific to each location. We can adjust the culture of the bivalve, but we can’t just adjust the dials on climate change,” said Filgueira.

As qualified marine researchers continue their quest to predict potentially dramatic shifts that climate change may have on the aquatic ecosystems, society will need to become aware, prepared, and able to make management decisions in favor of a flourishing marine ecosystem.

Dr. Filgueira’s research findings have strength in observation and prediction, using an ecosystem modelling approach, to determine likely and unlikely consequences of climate change drivers that impact ecosystem functioning.

Oyster Culture (Photo Credit: Luc Comeau)

The full results of this research were recently published in the journal Global Change Biology. Thomas Guyondet, Research Scientist, Department of Fisheries and Oceans; Luc A. Comeau, Research Scientist, Department of Fisheries and Oceans; and, Réjean Tremblay, Aquaculture Professor, Université du Québec join lead author Dr. Filgueira in this academic publication.